Without limiting the scope of the present invention, its background will be described in relation to exploratory subterranean well operations, as an example.
In the search for oil reserves in unproven areas, exploratory wells are often drilled for the purpose of extracting geological, geophysical or geochemical information about an area with a view to discovering and exploiting new reserves. In such operations, after an oil or gas well has been drilled, it is desirable to determine the flow rates of the various constituents of the well fluid. For example, the well fluid may be a two-phase, three-phase or four-phase fluid including oil, gas, solids and water.
Typically, the well fluid stream is flowed from the well through a surface test tree at the wellhead. This fluid stream then passes through a choke manifold including one or more valves which are used to accurately throttle the flow from the well so that the fluid pressure downstream from the choke manifold is reduced. The fluid stream is then flowed into a processing facility including a heater and a separator, both sized to accommodate the appropriate flow. The heater is used to maintain the fluid stream at a temperature suitable to enable the desired fluid mobility. This is particularly important in fluid streams containing heavy oils, such as oils having a specific gravity below 22.3° as classified by the American Petroleum Institute. The separator separates the fluid into its constituents such as gas, oil and water constituents. The separated constituents are flowed out of the separator in individual lines. The flow rate of the constituent in each line is measured, so that the operator will know the relative amounts of each. After these measurements are made, the gas constituent is typically flared, the water constituent is directed to a treatment or disposal location and the oil constituent is pumped to a burner for elimination.
It has been found, however, that due to the high viscosity of the oil constituent, particularly in fluid streams having heavy oils, the mobility of the oil constituent may be unacceptably low to enable pumping to the burner. One way to improve the mobility of the oil constituent is to add a lighter hydrocarbon liquid, referred to herein as diesel, to the oil constituent to generate a mixture having a lower viscosity than the original oil constituent such that the thinned oil can be more easily transported to the burner. It has been found, however, that mixing diesel into the oil constituent is wasteful of the diesel as it is burned with the oil constituent without delivering any useful work. In addition, use of diesel for mixing with the oil constituent requires expensive transportation and storage of the diesel which is particularly significant when the exploratory well is drilled at a remote, inaccessible location.
Accordingly, a need has arisen for an apparatus and method for determining the flow rates of the constituent fluids in a multiphase well fluid stream from an exploratory well. A need has also arisen for such an apparatus and method that is operable to maintain suitable mobility of the oil constituent even in fluid streams having heavy oils while minimizing or eliminating the requirement of mixing diesel into the oil constituent.